A number of prior art measuring systems exist in which the power, axis, prism and other characteristics of ophthalmic lenses are measured at local points on the lens. Commercial instruments for performing this are available from Humphrey Instruments Corporation of San Leandro, Calif., who produce an automatic lensmeter based on the four beam Hartmann method, from the Nidek Company, of Gamagori, Japan, who use dynamic positioning of the detector to locate the focal position of the probe beam, and the Topcon Company of Tokyo, Japan whose instrument also uses dynamic positioning.
The prior art Hartmann measurement of spectacle lens power at a localized area on the lens uses four measurement beams. The four beam technique is used in all of the previously available commercial instruments for measuring ophthalmic lens power, such as those mentioned above. The technique is described in an article by D. and Z. Malacara entitled "Testing and centering of lenses by means of a Hartmann test with four holes" published in Optical Engineering, Vol. 31, No. 7, pp. 1551-1555 (July 1992). Since it is difficult to discriminate between the separate spots resulting from the four beams if detection is made at the focal plane of the lens under test, one technique used is to detect the position of the beams at a defocused position. Since this introduces inaccuracies into the measurement, it is preferable to measure the four beams at the focal plane, but these measurements must then be performed separately and sequentially.
This can be achieved by a number of methods, four of which are described by the Malacara's. They propose using either (i) a rotating chopper which exposes only one beam at a time, or (ii) a laser beam scanner which projects beams sequentially, or (iii) four separate light sources (such as LEDs), with a small circular diaphragm at the focus of the lens, the separate sources being lit and extinguished sequentially, or (iv) separate light sources as in method (iii) but with small lenses in front of each of them.
The four-beam technique enables the determination of two radii of curvature, and of one angle of reference in the lens. This is sufficient for measuring the power of spherical lenses anywhere on their surface, and to determine lens decentering or coma. It is also useful for measuring the power of aspheric lenses near their optical center. However, it cannot detect spherical aberrations of second order or higher, since there are more parameters to be measured than beams. Therefore, off-axis measurements such as the high order spherical aberration, or the coma of complex lens forms, which require higher order Zernicke polynomial analysis, cannot be performed using the prior art four-beam Hartmann methods.
Such measurements can be performed by instruments developed for mapping the optical characteristics of the whole surface of ophthalmic lenses, which also thereby enable such characteristics as toric axis and corridor direction to be determined on more complex lenses. Commercial instruments for performing this are available from Rotlex 1994 Ltd., of Dimona, Israel, whose instrument is based on the Moire effect, and enables mapping of optical elements by means of measurement of the Moire Deflectometry effect of the grating, from Automation and Robotics S. A. of Brussels, Belgium, who measure the deformation of a grid, and from the assignees of the current invention, Visionix Limited of Jerusalem, Israel, whose instrument is based on a computerized Hartmann measurement using a large number of beams, as described in the copending published PCT Application No. PCT/EP95/02283 (Publication No. WO95/34800) mentioned above.
Prior art methods for measuring the optical characteristics of complete pairs of spectacles are limited to measurement of the distance between the optical centers of the two lenses within their frames, known as the inter-pupil distance of the lenses, or the distance from the optical center of each lens to the bridge center line. These measurements can be performed using a number of prior art instruments, such as that offered by Humphrey Instruments Incorporated of San Leandro, Calif., which is based on U.S. Pat. No. 4,098,002, by Campbell et al., and that offered by the Nidek Company Limited of Gamagori, Japan, which is based on U.S. Pat. No. 5,152,067, by Kurachi et al.
On the other hand, the full optical characteristics of each lens are only measured separately and independently of their mounting position in the spectacle frames, by one of the above mentioned mapping instruments. This is a serious limitation of the prior art methods, since even though each individual lens may be correctly manufactured, and the lenses mounted with the correct inter-pupil distance, they may be incorrectly cut, asymmetrically mounted or incorrectly orientated within the frame. none of which faults are detectable by the prior art systems. Such errors are a serious cause of user fatigue and non-tolerance. This phenomenon is especially prevalent with progressive lenses, which have to be aligned very accurately to be comfortable and useable. Inter-pupil distance measurement, as performed by any of the presently available instruments, does not detect any of these faults.
Furthermore, none of the available prior art inter-pupil measurement instruments mentioned above are able to measure the height of the optical center, or in the case of a progressive lens, optical centers, with respect to the spectacle frames. This distance is also important to ensure comfort in the use of the spectacles. For progressive lenses, accurate measurements of these distances are very critical for correct use of the spectacles.
The alignment of progressive lenses, and to a lesser extent, that of toric lenses too, is so critical that during all stages of their manufacture, marks are made on their surface to define the correct alignment of the optical axes, corridor and center. Since these marks are virtually or completely obliterated during various stages of the manufacturing process, they have to be remade several times during manufacture. The result of these repeated remarkings can manifest itself as a cumulative position and alignment error of the markings with respect to the true optical characteristics of the lens. As a result, when the final lens is cut and mounted into the spectacle frames according to the markings on it, even though it has been manufactured accurately, incorrect marking can be the cause of serious misalignments, both lateral and angular, with respect to the prescribed requirements.
A research program into the accuracy with which spectacles with progressive lenses fulfill the prescribed requirements has been undertaken by the Ophthalmic Appliances Testing Service of the Department of Optometry and Visual Science, City University, London. A preliminary report published in Optical World, May 1997, pp 26-27, has shown, for instance, very large deviations of up to 5 dioptres in the base down prismatic power of the lenses of identically prescribed spectacles. Since, as mentioned previously, errors such as these in fulfilling prescription requirements cause fatigue and non-tolerance for the user, this problem is of a serious nature, and has not been solved by previously available lens measurement and marking systems.
Other conventional ophthalmic measurement systems are described in the following publications:
Published European Patent Application No. 95104527.7 (Publication No. 0 676 629 A2) to Zeiss describes apparatus for measuring the index of refraction of spectacle lenses without measuring the surface geometry of the glass.
U.S. Pat. No. 5,175,594 to Campbell describes a lensmeter with correction for refractive index and spherical aberration.
U.S. Pat. No. 5,307,141 to Fujieda describes apparatus for measuring the refractive characteristics of a spectacle lens.
U.S. Pat. No. 5,339,151 to Shinn describes a spectrometer incorporated to a conventional lensometer.
U.S. Pat. No. 5,469,261 to Hellmuth et al describes a method and apparatus for measuring curvature of lens surfaces, physical thickness of lens, and the index of refraction of the lens.
Copending Published PCT Application No. PCT/EP95/02283 (Publication No. WO 95/34800) describes apparatus for mapping an optical element, the apparatus including a light source arranged to transmit a light beam toward the optical element, a beam separator including a plurality of beam separating elements operative to separate the light beam into a corresponding plurality of light beam portions, an optical sensing device operative to generate a light spot map including a plurality of light spots corresponding to the plurality of beam separating elements and an optical element characteristic computation device operative to derive at least one characteristic of the optical element from the light spot map and including apparatus for identifying the beam separating element corresponding to an individual spot based at least partly on information other than the location of the spot.
The focimeter is a conventional device for measuring optical characteristics of spectacle lenses. The focimeter measures the focal length at any point on each of the spectacles' lenses.
The disclosures of all publications mentioned in this section and in the other sections of the specification, and the disclosures of all documents cited in the above publications, are hereby incorporated by reference.